Lightguide fiber, which is commonly being used in many communication applications, is manufactured by drawing out a lightguide preform, comprised of a refractive core surrounded by glass cladding. Lightguide preforms are commonly fabricated by one of several well known chemical vapor deposition processes. One such process is the Vapor Phase Axial Deposition (VAD) technique wherein a vertically aligned rod known as a bait rod is rotated about its axis with the lower end thereof within the combustion zone of a gas torch. Precursor gases, such as SiCl.sub.4 and GeCl.sub.4, are flowed through the torch into the combustion zone so that submicron-sized glass particles, formed by the reaction product of the gases, are deposited upon the end of the bait rod. The bait rod is slowly withdrawn as the deposition process continues, causing a glass soot boule to be grown endwise thereon. The soot boule derives its name from the fact that collection glass particles comprising the boule appear as soot particles.
After the soot boule is grown in the above-described manner, the boule is consolidated, typically by sintering. Following consolidation, the boule is then cladded by insertion into a glass tube which is then collapsed thereabout to form the preform. In certain instances, the boule is initially cladded by deposition of pure silica glass thereon prior to consolidation and insertion into the silica glass tube.
To obtain lightguide fiber having the desired transmission characteristics and mechanical flexibility, the mass ratio of the core to the cladding of the preform from which the fiber is drawn must be within a narrow range of values. Typically, boules grown by the VAD process are too large in their original state to fabricate a preform having the desired core-to-cladding mass ratio. Rather, the boule is usually stretched before insertion into the silica tube to reduce its mass per unit length. Stretching is accomplished on a glass lathe whose construction is well known in the art. The boule is mounted between the headstock and tailstock of the lathe for cojoint rotation therewith. As the boule rotates, a torch moves underneath the boule along the axis thereof at a constant rate towards the headstock. Simultaneously with the movement of the torch, the tailstock moves away from the headstock, causing the boule to be stretched to reduce the diameter thereof.
In order for the stretched boule to reach its equilibrium shape, the heated glass deforms or flows in response to the tensile force applied during stretching, causing fluctuations in the diameter of the boule. The diameter variations occur because the stretched diameter of the boule almost always overshoots the equilibrium shape. If an undersized portion of glass leaves the zone heated by the torch, conservation of mass requires that an oversized portion follow it, resulting in variations in the boule shape. Diameter fluctuations in the stretched boule causes variations in the core-to-cladding mass ratio of the resultant preform which result in degradation of the transmission characteristics of the drawn fiber that are undesirable. Although efforts are made during stretching to reduce the diameter variations, fluctuations of up to 30% are not uncommon especially at the inception of stretching. While the diameter fluctuations do ultimately dampen out, much of the boule is drawn before steady state conditions are reached.
Accordingly, there is a need for a method for stretching a glass rod, such as a lightguide boule, which affords increased control over diameter variations.